Copper interconnects can be utilized to carry electricity in microcircuits. Copper is, however, subject to electromigration. Electromigration can degrade the performance of copper interconnects, for example by aiding the growth of voids in the interconnect. As a result, copper interconnects may be more subject to failure. The resistance of copper to electromigration is strongly dependent upon the crystal structure of the copper interconnect.
Conventional processing of copper interconnects can include an annealing step used to control the crystal structure of the copper. The structure and grain size of the copper depends upon when annealing takes place. One conventional method for processing copper interconnects anneals the interconnect after the copper film is deposited and before polishing removes excess copper. This annealing step can result in the formation of grain boundaries which have grain boundary triple points through the interconnect. A grain boundary triple point is an intersection of grain boundaries. It is known that grain boundaries are paths of high diffusion for copper atoms. It is also known that the grain boundaries with grain boundary triple points are sites for high electromigration because the grain boundary triple points connect paths of high diffusion. As a result, a copper interconnect formed by this process is subject to relatively high electromigration.
A second conventional method for processing copper interconnects anneals the interconnect after the copper film has been polished and passivated. As a result, the copper interconnect may have grain boundaries with very few grain boundary triple points. When copper has this structure, it is said to have a bamboo structure. Consequently, electromigration due to grain boundaries having grain boundary triple points is reduced. However, the orientation of the grains is not controlled. As a result, some grain boundaries have a high defect density and are likely to be high diffusion paths. As a result, the copper interconnect is still subject to electromigration.
Accordingly, what is needed is a system and method for providing a copper interconnect having reduced electromigration. The present invention addresses such a need.